1. Field of the Invention
The present invention relates generally to four-wheel drive vehicles and particularly to four-wheel drive vehicles switchable between two-wheel drive and four-wheel drive.
2. Description of the Background Art
Some four-wheel drive vehicles adopt a so-called part time 4WD system allowing switching between a two-wheel drive mode suitable for typically running on a significantly safe, dry road and excellent in mileage and a four-wheel drive mode suitable for running in the rain and/or snow or on a bad road and the like.
Japanese Patent Laying-Open No. 03-148337 discloses a drive state switching device provided in a vehicle switchable between such four-wheel drive and two-wheel drive and actuated by an actuator.
When the drive state switching device switches a drive state, the device requires a period of time, which significantly depends on the viscosity of a working fluid in a case housing the device. However, such devices disclosed in documents including Japanese Patent Laying-Open No. 03-148337 are actuated by actuators actuated on a current having a fixed value.
Typically, sports utility vehicles (SUVs), pickup trucks and other similar vehicles have a two-wheel drive mode, a four-wheel drive mode and a variety of other drive power transfer modes switched by a mechanism configured in a transfer. The “transfer” as referred to herein indicates is a portion extracting a torque from a powertrain of a drive system serving as a base (e.g. FF, FR) to a side that is originally not a driving wheel, i.e., a portion extracting a torque transferred from a powertrain associated with a full time driving wheel to a driving wheel which is not a full time driving wheel.
The switching mechanism provided in the transfer requires a disadvantageously long period of time to switch in winter and/or in cold districts in particular as external temperature varies and accordingly the transfer's interior, a front diff configuring a driving system, and the like vary in drag resistance and the like. This is attributed to the fact that at low temperature the lubricant oil increases in viscosity and the variation in drag resistance of the transfer, the front diff and the like is significantly affected by the variation in viscosity of the lubricant oil. In addition, a motor is used as an actuator producing thrust required for switching has a rotation shaft support fitted with a precision varying with temperature and the rotation shaft slides against varying resistance, and such also affects the switching.
As such, if in winter a vehicle set in the two-wheel drive state in the previous day is started by a driver in the following morning and immediately thereafter the driver operates a switch to switch the vehicle from two-wheel drive to four-wheel drive, the vehicle may not readily switch to four-wheel drive.